One of various possibilities to achieve ply bonding between at least two plies of tissue paper without the use of glue is disclosed in WO-A-99/33646. The known device comprises two rollers forming a nip through which at least two plies which are to be bonded are fed. At least the outer periphery of one of the rollers is entirely covered with abrasive material such as the material used for sandpaper so as to achieve an irregular rough surface. This abrasive material is pressed into the nipped plies, whereby ply bonding is achieved.
However, an irregular rough surface structure is imprinted into at least one of the plies over the entire surface. Therefore, the outer appearance of the combined plies is irregular or the ply bonding is (almost) not visible. In addition, it will not be possible to create volume between the plies by embossing and even pre-embossed webs with a defined thickness would be flattened by compressing of the ply.
To enhance the visual appearance of the bonded plies, WO-A-99/33646 additionally suggests a subsequent embossing step. The subsequent embossing requires additional devices with the associated additional steps. This, in turn, increases the complexity of the apparatus and, hence, the manufacturing costs of the final product.